1. Field of the Invention
The invention relates to fasteners having interlocking elements on adjacent edges of respective tapes which can be opened and closed by a slider, and to methods for making the fasteners.
2. Description of the Prior Art
Prior art slide fasteners, such as fasteners utilizing spiral coupling members, coil or meander ladder coupling members, plastic or metal molded scoop coupling elements, or the like, are sometimes defective due to variation in the spacing of coupling elements or head portions along the length of the fastener; elements spaced too far apart decrease the crosswise strength of the fastener, and elements spaced too closely together result in increased difficulty in slider movement and excessive rigidity. Also, variation in the spacing of coupling elements makes it difficult to match stringers, i.e., stringers with different spacing of coupling elements interlocked together produce a curved slide fastener rather than a straight fastener.
The problem of variation in spacing of the engaging elements of coupling members is particularly acute in spiral slide fasteners wherein the interlocking of the engaging elements results from the twist imparted to portions of the oblate cross-sectional filament coiled to form each coupling member. The engaging surfaces of interlocked spiral coupling members have only a small incline to the direction of crosswise forces and thus are subject to being pulled apart by crosswise forces particularly if the spacing between engaging elements is too large. U.S. Pat. No. 2,541,728 discloses an apparatus and method of making interlocking coils wherein convolutions of the coils wound on mandrels are formed by bending and twisting of the filament and notching of the edges of the filament loops by the pressure of one convolution on another as the interlocked coils are moved by longitudinal forces along the mandrels through a heating element; such notching and close spacing of interlocking coils result in decreased ease of operation in slide fasteners.